In recent years, the efficiencies of automotive engines have increased significantly in order to conserve fuel and to comply with statutory and regulatory requirements on automotive fuel consumption. This increased efficiency has, in turn, led to more severe service requirements for the engine lubricants because the higher efficiencies have generally been accompanied by higher engine temperatures as well as higher bearing pressures concomitant upon the use of higher compression ratios. These increasingly severe service requirements have made it necessary for lubricant manufacturers to provide superior lubricants. Furthermore, it is expected that this trend will continue and that in the future even more severe service ratings will be established by engine manufacturers. At present, the API "SH" rating is currently employed for passenger car motor oils for gasoline engines and this represents a significant increase in the service requirements of lubricants. Thus, there is a continuing need for lubricants with superior performance characteristics.
One of the performance characteristics which is of greatest significance is the viscosity index (VI). This represents the extent to which the viscosity of a lubricant varies with temperature. Lubricants of high VI change relatively little in viscosity as temperature increases, at least as compared to lubricants of lower VI. Since retention of viscosity at higher temperatures is a desirable characteristic, high viscosity index is desirable. Satisfactory viscosity properties may be conferred either by suitable choice of the lube base stock or by the use of VI improvers which are generally high molecular weight polymers.
The extent to which VI properties can be varied by the use of these improvers is, however, limited because not only are large amounts of improver expensive but the improvers are subject to degradation in use so that service life of lubricants containing large amounts of improver may be limited. This implies that improvements in the VI of the base stock are desirable.
Synthetic lubricants produced by the polymerization of olefins in the presence of certain catalysts have been shown to possess excellent VI values, but they are expensive to produce by the conventional synthetic procedures and usually require expensive starting materials. There is, therefore, a need for the production of high VI lubricants from mineral oil stocks which may be produced by techniques comparable to those presently employed in petroleum refineries.
Studies to date have shown that lubricants prepared via the hydroisomerization of Fischer-Tropsch wax, are equivalent to polyalphaolefins (PAO) except in low temperature performance and base oil oxidative stability. Therefor, a process is needed which is capable of increasing the oxidative stability of hydroisomerized Fischer-Tropsch waxes while producing a lubricant having a high viscosity index (VI).